1. Field of the Invention
The present invention relates, in general, to film bulk acoustic resonator filters having an unbalanced-balanced input/output structure and, more particularly, to a film bulk acoustic resonator filter having an unbalanced-balanced input/output structure, which improves the implementation of the mass production of the filter while enabling a balun circuit unit for converting between an unbalanced signal and a balanced signal and a filter circuit unit for filtering an input signal to be implemented on a single chip using simpler processing.
2. Description of the Related Art
Currently, in the case of an RX filter provided in a Radio Frequency (RF) transceiver, such as a Code Division Multiple Access (CDMA) system or a Global System for Mobile communications (GSM), an antenna end connected to an input terminal of the RX filter has an unbalanced structure, and a receiving end connected to an output terminal of the RX filter requires a balanced signal. Therefore, the RX filter requires an unbalanced-balanced input/output structure.
For an RX filter suitable for this requirement, a SAW filter comprised of an acoustically coupled Surface Acoustic Wave (SAW) filter has been broadly used, and is advantageous in that it can be easily implemented using a balanced input/output, unbalanced input/output or balanced-unbalanced input/output structure.
However, as the use of wireless communication has recently increased, a Film Bulk Acoustic Resonator (FBAR) filter, which can be easily implemented with a Monolithic Microwave Integrated Circuit (MMIC), be operated at an ultrahigh frequency of 15 GHz, and realize super light weight and microminiaturization, has been researched as an advanced device.
An FBAR resonates at a certain frequency band by evaporating Zinc Oxide (ZnO) or Aluminum Nitride (AIN), which is a piezoelectric material, on a silicon (Si) or Gallium Arsenide (GaAs) wafer using an RE magnetron sputtering method and causing a piezoelectric phenomenon. Further, if an input is applied to an upper or lower electrode, a Bulk Acoustic Wave (BAW) is generated due to a piezoelectric effect. Further, if the frequency of the BAW becomes equal to that of an input electrical signal, resonance occurs. Further, resonators using this resonance are electrically coupled to each other, so that an FBAR filter and even an FBAR duplexer can be implemented.
RF devices using the FBAR have several advantages, in that filters using various frequency bands, such as a local area communication filter (2 GHz) for Bluetooth and a filter for Global Positioning System (GPS) communication, can be manufactured in addition to a filter for International Mobile Telecommunication (IMT)-2000 band, a maximum operational frequency band can be extended up to 15 GHz, and a conventional semiconductor wafer is used and the RF devices can be integrated with other active devices, so that a frequency control circuit can be completely implemented with an MMIC. However, an FBAR filter is generally manufactured in the form of a ladder-type filter having an unbalanced input-unbalanced output structure or a lattice-type filter having a balanced input-balanced output structure. The FBAR filter is advantageous in that it is difficult to implement an FBAR filter having an unbalanced-balanced input/output structure.
Therefore, in order to use such an FBAR filter as an RX filter placed between an antenna end and a receiving end, a Balun transformer for converting between an unbalanced signal and a balanced signal is required. Further, in order to realize miniaturization, various research into integrating the Balun transformer and the FBAR filter into a single chip have been conducted.
As an example of the research, Lakin shows that, when the input and output of a Coupled Resonator Filter (CRF) that is comprised of two FBAR resonators layered in a longitudinal direction and a coupling layer interposed between the resonators to control the acoustic coupling thereof are electrically separated, the CRF can be used as a balun. U.S. Pat. No. 6,670,866 proposes an FBAR balun using the above method.
FIG. 1 is a sectional view showing the structure of an FBAR balun proposed in U.S. Pat. No. 6,670,866. As shown in FIG. 1, the conventional FBAR balun includes a first electrode 12a connected to a balanced signal output end 11c, a first piezoelectric layer 13a formed on the first electrode 12a, a second electrode 12b that is formed on the first piezoelectric layer 13a and grounded, a dielectric layer 14 formed on the second electrode 12b, a third electrode 12c that is formed on the dielectric layer 14 and connected to a balanced signal output end 11b, a second piezoelectric layer 13b formed on the third electrode 12c, and a fourth electrode 12d that is formed on the second piezoelectric layer 13b and connected to an unbalanced signal input end 11a. 
As described above, the conventional FBAR balun is constructed in a structure in which a first resonator 16 comprised of the first electrode 12a, the first piezoelectric layer 13a and the second electrode 12b, and a second resonator 17 comprised of the third electrode 12c, the second piezoelectric layer 13b and the fourth electrode 12d are sequentially stacked, with the dielectric layer 14 placed therebetween. In addition, in order to improve unbalanced parasitic characteristics, a compensation capacitor 15a is connected between the second electrode 12b and the fourth electrode 12d. Further, in order to increase the bandwidth between the two resonators 16 and 17, a coupling inductor 15b is connected between the first and second electrodes 12a and 12b, and a coupling inductor 15c is connected between the third and fourth electrodes 12c and 12d. 
Further, as shown in FIGS. 2a and 2b, the FBAR balun 10 is coupled to a ladder-type FBAR filter 24 or a lattice-type FBAR filter 25, so that the FBAR balun 10 and the filter 24 or 25 can be implemented on a single chip. Therefore, when a filter having an unbalanced-balanced input/output structure is implemented, the size of the filter can be greatly decreased while the manufacturing process thereof can be simplified.
However, the above FBAR balun has a fatal disadvantage in that, since it is difficult to obtain a target yield, the implementation of the mass production of the FBAR balun greatly decreases. In detail, a typical FBAR resonator is a layered structure including an electrode layer, a piezoelectric layer and an electrode layer, and the operational frequency thereof is determined by the thickness of each layer. Therefore, in order to obtain the target yield of the FBAR resonator, in-wafer uniformity and wafer-to-wafer uniformity of the thicknesses of respective layers must be guaranteed. However, since the technical precision of current equipment for forming layers is not sufficient to guarantee uniformity of the thicknesses of respective layers, it is impossible to guarantee a yield meeting a target level. In order to improve upon this disadvantage, the yield of frequency is improved through a frequency adjustment process after devices are manufactured, at the time of manufacturing an FBAR resonator. However, since the above-described multi-layered FBAR balun has a very complicated structure comprised of two piezoelectric layers, four electrode layers and one dielectric layer, it is almost impossible to guarantee uniformity of the thicknesses of respective layers to a desired target level even though the frequency adjustment process is added. Therefore, it is actually impossible to achieve a target yield sufficient to guarantee the implementation of the mass production or the marketability of the above-described two-layered FBAR balun.
Moreover, the above-described FBAR balun has problems, such as the control of internal stress caused by a multi-layered structure, the guarantee of the orientation of the second piezoelectric layer 13b, and the decrease of the amount of production caused by the slow speed of formation of the first and second piezoelectric layers.